Rifle length will give you more options for springs, buffers, spacers, etc but compactness usually rules that out.
With full power loads it will be hot and you'd want the most weight you can run. Drops on subsonic 220s, 240s zeroed at 100yds is about 36" at 200 yds.
On my pistol 10" and carbine 16" I run a h3 buffer and cut down spring in Carbine tubes with 240 subs and burn rate around AA1680, 5744. I only runs Subs with 300blk.
If I want supersonic I use 7.62x40WT to get most velocity.